This invention relates to an apparatus and method for controlling the gain applied to a far-end audio signal in bidirectional voice communications.
In telephony, audio signals (e.g. including voice signals) are transmitted between a near-end and a far-end. In a bidirectional voice communication, the “near-end” and “far-end” are defined relative to each participant. Thus, the “near-end” for one participant will correspond to the “far-end” for the other participant. Far-end signals which are received at the near-end may be outputted from a loudspeaker at the near-end. A microphone at the near-end may be used to capture a near-end signal to be transmitted to the far-end, such as a voice of a participant at the near-end. An “echo” occurs when at least some of the far-end signal outputted by the loudspeaker at the near-end is included in the near-end signal which is transmitted back to the far-end. In this sense, the echo may be considered to be a reflection of the far-end signal.
An example scenario is illustrated in FIG. 1, which shows a signal being captured by a far-end microphone and output by a near-end loudspeaker. The echo is a consequence of acoustic coupling between the loudspeaker and a microphone at the near-end; the near-end microphone captures the signal originating from its own loudspeaker in addition to the voice of the near-end speaker and any near-end background noise. The result is an echo of the far-end signal at the far-end loudspeaker. Echo cancellation is an important feature of telephony. Hands-free devices and teleconferencing, in particular, require echo cancellation that can adapt to environments having a wide range of acoustic characteristics. In these examples, a combination of factors contributes to echo being more of an issue. First, the volume at which the far-end signal is outputted from the near-end loudspeaker is typically loud enough that the far-end signal is a significant part of the signal captured by the near-end microphone. Second, the physical arrangement of the loudspeaker and microphone in these types of arrangements tends to result in a good acoustic coupling between the two.
Acoustic echo cancellers typically synthesise an estimate of the echo from the far-end voice signal. The estimated echo is then subtracted from the microphone signal. This technique requires adaptive signal processing to generate a signal accurate enough to cancel the echo effectively. An adaptive filter is often used to model the environment's acoustic impulse response. The adaptive filter is often followed by a non-linear processor (NLP) for removing any residual echo.
The performance of the echo canceller depends on the platform, and particularly on the audio interface, the interface driver and related hardware, the pre and post amplifier (if any), and characteristics of microphone and speaker. It is challenging to achieve full duplex voice communication on a wide variety of platforms with sufficient echo cancellation. The echo cancellation can be particularly challenging on platforms that are highly non-linear. The processing carried out in order to cancel echoes in highly non-linear platforms can be resource intensive and, in some situations, such as at times of double talk, cause cancellation of the near-end microphone speech along with the echoes, which results in severe degradation in the quality of a near-end signal received at the far-end, e.g. during double talk.
Therefore, there is a need for a method of improving the quality of the near-end signal when implementing acoustic echo cancellation.